The present invention generally relates to duplex units and image forming apparatuses having a duplex unit, and more particularly to a duplex unit for reversing the side of a recording sheet and an image forming apparatus having such a duplex unit.
Recently, some image forming apparatuses such as laser printers, copying machines and facsimile machines are provided with a duplex unit which is used for reversing the side (or face) of a recording sheet so that an image can be formed on both sides of the recording sheet. In the laser printer, for example, the duplex unit is provided adjacent to a recording part and reverses the side of the recording sheet which has a first side already recorded with an image in the recording part so that the recording part can record an image on a second side of the recording sheet.
The duplex unit operates in a through mode or a stack mode. In the through mode, the duplex unit reverses the side of the recording sheets respectively recorded with an image on the first side thereof and successively supplies the reversed recording sheets to the recording part so that an image can be recorded on the second side of each of the recording sheets. On the other hand, in the stack mode, the duplex unit reverses the side of the recording sheets respectively recorded with an image on the first side thereof and stacks the reversed recording sheets. The stacked recording sheets are then successively supplied to the recording part so that an image can be recorded on the second side of each of the recording sheets.
The duplex unit which operates in the through mode has an advantage in that the recording speed (that is, the printing speed in the case of the printer) can be increased because the reversed recording sheets are continuously supplied to the recording part. However, there is a drawback in that the reversed recording sheets cannot be stopped temporarily during the duplex recording operation when it is necessary to wait for an image information to arrive from a controller. For this reason, there is a need from the practical point of view to provide a temporary stacking part in order to temporarily stack the reversed recording sheets, but this would increase both the overall size and production cost of the image forming apparatus.
The duplex unit which operates in the stack mode may be categorized into a serial stack type and a parallel stack type. In the serial stack type duplex unit which operates in the stack mode, the reversed recording sheets are stacked serially, that is, generally along the length of the recording sheets. Hence, there is a need to provide a transport path of a predetermined length to enable the serial stacking of the recording sheets. In addition, since the predetermined length is determined depending on a standard size (or maximum size) of the recording sheets, a space must be provided between a rear end of one recording sheet of the standard size and a front end of another recording sheet of a size smaller than the standard size. Such a space must be provided when recording sheets of different sizes are used simultaneously, and this space deteriorates the recording speed. Therefore, there is a limit to increasing the recording speed.
On the other hand, in the parallel stack type duplex unit which operates in the stack mode, the reversed recording sheets are stacked in parallel, that is, one on top of another. The stacked recording sheets are then successively supplied to the recording part in a sequence opposite to the stacking sequence. In other words, the recording sheets are successively supplied to the recording part from the top of the stack. For this reason, there is a need to provide a memory having a large memory capacity for storing at least two times a data quantity of the image data with respect to all of the recording sheets in the stack, and there is a drawback in that the image forming apparatus becomes expensive. Furthermore, because the last reversed recording sheet at the top of the stack must first be supplied to the recording part, there is a time loss between a time when the last reversed recording sheet is stacked and a time when the image is recorded on this last reversed recording sheet.
In most cases, the front end of the reversed recording sheet which has the image recorded on the first side thereof and is supplied from the duplex unit to the recording part for recording an image on the second side thereof corresponds to the rear end of the recording sheet before the side reversal. In other words, when recording the image on the second side of the recording sheet, the image data is supplied to the recording part in a reverse sequence to the recording sheet, that is, starting from the last line, so that the recording direction matches for the first and second sides of the recording sheet.
However, the recording sheet is heated when passing a fixing part and the recording sheet slightly shrinks when heated. When the recording is carried out on the second side of the reversed recording sheet which is still shrunk, the image on the second side becomes stretched compared to the image on the first side after the heated recording sheet returns to the normal state from the shrunk state. As a result, there are problems in that the starting or top positions of the images on the first and second sides differ and top margins (or blank portions) differ between the first and second sides of the recording sheet.
Conventionally, the front end of the recording sheet and the front end of the image are matched at the same position for both the first and second sides of the recording sheet. Hence, as shown in FIG. 1, a front end 2B.sub.1 of an image 2B on the second side of a recording sheet 2 and a rear end 2A.sub.2 of an image 2A on the first side of the recording sheet 2 are matched. In other words, end portions 2D of the images 2A and 2B are matched on the recording sheet 2. However, when the recording sheet is heated and shrunk, a front end 2A.sub.1 of the image 2A and a rear end 2B.sub.2 of the image 2B do not coincide on the recording sheet 2. That is, end portions 2C of the images 2A and 2B differ by a length L1 which corresponds to the shrinkage. The shrunk recording sheet 2 returns to the normal state after a sufficient time elapses, and there is a problem in that the image 2A becomes expanded compared to the image 2B and the problems described above occur.
It is conceivable to eliminate the above described problem by changing the transport speed of the recording sheet for the recording on the first side and the recording on the second side, or by changing a write clock signal which is used to control the recording timing so as to use different magnifications for the first and second sides. However, a complex circuitry is needed to take such conceivable measures, and the image forming apparatus will become considerably expensive.